Security systems are presently limited in their ability to detect contraband, weapons, explosives, and other dangerous objects concealed in cargo. It is known in the art that images of various types of material can be generated by using various X-ray scattering and transmission techniques. The intensity of transmitted X-rays is related to the thickness, density and atomic number (Z) of the material scattering or absorbing the X-rays. Materials with high atomic number (Z>70) are characterized by the high attenuation of x-rays having energies in the high end of the X-ray spectrum, and in particular, energies in the range of 2-10 MeV, due to a process called e+/e− pair production. Therefore, X-ray transmission images are, in part, modulated by variations in the atomic number of items of various materials inside the cargo.
As a result of the image modulation due to the density, thickness and atomic numbers of various materials, it is common for X-ray imaging systems to produce images with dark areas. These dark areas might be indicative of the presence of threat materials; however, they yield little information about the exact nature of threat. In addition, radiographic images produced by conventional X-ray systems are often difficult to interpret because objects are superimposed. Therefore, a trained operator must study and interpret each image to render an opinion on whether or not a target of interest, or a threat, is present. Operator fatigue and distraction can compromise detection performance when a large number of such radiographic images are to be interpreted, such as at high traffic transit points and ports. Even with automated systems, it becomes difficult to comply with the implied requirement to keep the number of false alarms low, when the system is operated at high throughputs.
One method of obtaining more useful information and clarity from X-ray imaging is by using dual-energy systems to measure the effective atomic numbers of materials in containers or luggage.
Typical X-ray inspection systems include an X-ray generator which comprises a heated cathode filament emitting an electron beam. The emitted electrons are accelerated towards a target. The electron beam strikes the target at a focal spot and some portion of the kinetic energy contained within the electron beam is converted into X-rays. At the focal spot, the X-rays are emitted in all directions from the target surface, where the intensity and energy of X-rays varies based on the angle with respect to the electron beam direction. The generated X-rays are allowed to leave a heavily shielded area in a predefined direction through a collimator. Current x-ray inspection systems are very heavy, largely due to the massive amounts of shielding required to create the predefined area in which produced x-rays area allowed to propagate, such as in the forward direction where the X-rays are used for radiography or other purposes.
A greater amount of shielding is required when using electron targets made of materials having a high atomic number (high-Z). In contrast, low atomic number (low-Z) targets have a much more forward-peaked angular distribution, making it possible to eliminate a substantial amount of shielding. However, because of this forward-peaked angular distribution, when large areas need to be scanned, such as in mobile cargo radiography, the X-rays produced from low-Z targets typically do not cover the vertical extent of the object adequately. In addition, mobile cargo inspection systems typically require an X-ray source optimized for weight and performance. Currently, weight is primarily determined by the required quantity of shielding materials.
Furthermore, while mobile systems are available to provide inspection capabilities at locations which are constrained for space, such systems are generally large, heavy, and lack maneuverability. As a result these systems can be difficult to deploy quickly, especially in urban areas and pose several disadvantages and constraints.
Accordingly, there is still a need for improved inspection methods and systems built into a fully self-contained, smaller and more mobile vehicle that can be brought to any site accessible by roads and rapidly deployed for inspection. Moreover, there is an additional need for methods and systems that require minimal footprint with respect to the radiation dose to the environment, while still performing inspection using a sufficient range of the radiation energy spectrum to encompass safe and effective scanning of light commercial vehicles as well as cargo containers and trucks.
Additionally, there is a need for a system and method with reduced shielding requirements, thereby reducing the overall weight of an x-ray source employed in an x-ray inspection system, such as a mobile cargo inspection system.
Further, in the case of X-ray sources employing low Z targets, which require less shielding material but have limited capability in scanning the full vertical length of the object, there is a requirement for systems and methods to enhance the vertical scanning capability of such X-ray sources. What are also needed are systems and methods for deflecting the central point of an X-ray beam towards areas of high density in the scanned object.
In addition, there is also a need for an integrated X-ray inspection system further comprising a secondary scanning system such as a neutron subsystem to improve the material separation capability of the system.